1. Field of the Invention
The present invention relates to an image sensing apparatus and imaging system.
2. Description of the Related Art
Image sensing apparatuses typified by a CMOS (Complementary Metal Oxide Semiconductor) image sensor have a pixel array where pixels are arrayed in the row and column directions. Some image sensing apparatuses use a pixel array including a light-shielded area (optical black area) where light-shielded pixels are arranged, and an effective area where unshielded pixels are arranged.
According to techniques disclosed in Japanese Patent Laid-Open Nos. 10-126697 and 2006-025146, a black-level reference signal is read out from the light-shielded area, and the black level of a signal read out from the effective area is corrected using the readout black-level reference signal.
Recently, as the number of pixels increases and the semiconductor micropatterning technique advances, the unit pixel tends to be shrinked. Along with this, each element included in the unit pixel is also shrinked.
For example, an amplification transistor serving as a MOS transistor for amplifying a signal corresponding to charges generated in a photoelectric conversion unit in a unit pixel can be shrinked.
Letting W be the gate width of the amplification transistor, L be the gate length, and Cox be the gate insulating film capacitance per unit area, it is known that noise generated in the amplification transistor is inversely proportional to the square root of (W×L×Cox). That is, as the amplification transistor is shrinked to decrease the gate width or gate length, noise generated in the amplification transistor increases.
If the unit pixel is shrinked by the techniques disclosed in Japanese Patent Laid-Open Nos. 10-126697 and 2006-025146, noise generated in the amplification transistor increases in the effective area and light-shielded area, as described above. Particularly when noise generated in the amplification transistor in the light-shielded area increases, the amount of noise included in a black-level reference signal read out from the light-shielded area increases. This may decrease the precision of processing for correcting the black level of a signal read out from the effective area by using the readout black-level reference signal.